Process for treating waste liquors

ABSTRACT

Closed circuit process for treating waste liquors, such as spent sulfite liquors from wood digestion by coking the liquors in the liquid phase under pressure under turbulent flow conditions to form coke and a clear effluent of low chemical oxygen demand. In the process, a heat exchange zone, a coking zone, a steam-and flue gas-producing zone, an absorbing zone, a wood digesting zone and a kilning zone are provided and interconnected by suitable conduits. Pressurized spent liquor is sent to the heat exchange zone where it absorbs heat from the coking zone effluent before coke separation, thereby absorbing also heat content contained in the coke. The coke is discharged to the kilning zone where it is dried by hot flue gases produced by burning previously dried coke in the steam producing zone. Sulfur dioxide containing warm, flue gas from the kilning zone is contacted in an absorbing zone with cool coker effluent to produce warm ammonium bisulfite solution which is passed to a wood digestion zone as well as cool flue gas substantially free of all air pollutants. Sulfur-containing gases produced by coking and wood digestion are burned in the steamproducing zone to manufacture sulfur dioxide for recycling. Use of a closed system reduces to a minimum odors in the digestion section of a pulp plant since all the odorous sulfur compounds are burned and the sulfur dioxide is recovered.

United States Patent [191 Hess et al.

[ 1 *Feb. 20, 1973 [54] PROCESS FOR TREATING WASTE LIQUORS [75] Inventors: Howard V. Hess, Glenham; Edward L. Cole, Fishkill, both of NY.

[73] Assignee: Texaco Inc., New York, N.Y.

[* Notice: The portion of the term of this patent subsequent to Jan. 26, 1988, has been disclaimed.

[22] Filed: July 17, 1970 [21] Appl. No.: 55,649

Primary ExaminerRobert L. Lindsay, Jr. Assistant Examiner-Alfred DAndrea, Jr. Att0rneyThomas H. Whaley and Carl G. Reis HOT EFFLUENT 4 SPENT SULFITE LIQUOR WARM FL DRY COKE [5 7 ABSTRACT Closed circuit process for treating waste liquors, such as spent sulfite liquors from wood digestion by coking the liquors in the liquid phase under pressure under turbulent flow conditions to form coke and a clear effluent of low chemical oxygen demand. In the process, a heat exchange zone, a coking zone, a steam-and flue gas-producing zone, an absorbing zone, a wood digesting zone and a kilning zone are provided and interconnected by suitable conduits. Pressurized spent liquor is sent to the heat exchange zone where it absorbs heat from the coking zone effluent before coke separation, thereby absorbing also heat content contained in the coke. The coke is discharged to the kilning zone where it is dried by hot flue gases produced by burning previously dried coke in the steam producing zone. Sulfur dioxide containing warm, flue gas from the kilning zone is contacted in an absorbing zone with cool coker effluent to produce warm ammonium bisulfite solution which is passed to a wood digestion zone as well as cool flue gas substantially free of all air pollutants. Sulfur-containing gases produced by coking and wood digestion are burned in the steam-producing zone to manufacture sulfur dioxide for recycling. Use of a closed system reduces to a minimum odors in the digestion section of a pulp plant since all the odorous sulfur compoundsare burned and the sulfur dioxide is recovered.

7 Claims, 1 Drawing Figure TO BOILER CLEAN COOL FLUE GAS TO STACK WARM PULPING LIQUOR HOT FLUE GAS PROCESS STEAM BOILER ASH TO FURTHER (SMELT) PROCESSING PROCESS FOR TREATING WASTE LIQUORS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for treating waste liquors such as spent wood pulping liquors by coking in the liquid phase under pressure.

2. Description of the Prior Art Copending application Ser. No. 780,081, now U.S. Pat. No. 3,593,742, issued July 27, 1971, discloses a process for the treatment of waste liquors from pulp and paper mills wherein waste liquors are heated above about 450 under pressure sufficient to prevent vaporization of water and form coke. However, said application discloses only the separation of coke from aqueous liquid effluent of low Chemical Oxygen Demand suitable for reuse in the process. Considerable heat may be lost in this process since the coke is removed from the coker effluent before the effluent is used to preheat the feed.

SUMMARY OF THE INVENTION It has now been discovered that waste liquors can be treated in a closed-cycle process in which the coke produced is used to produce more steam and flue gases which are reused in the process and where heat exchange is brought about between the waste liquor charge and the effluent from the coker before coke separation.

DESCRIPTION OF THE INVENTION The process of this invention will be better understood by reference to the accompanying FIGURE showing diagrammatically a preferred embodiment of the present invention as applied to the treatment of waste liquor from an ammonia-based sulflte pulp mill.

With reference to the FIGURE, spent sulflte liquor is pumped through a pump up to a pressure of around 1,000 psig and passed through a heat exchanger where it is in heat exchange with hot coker effluent and thence to a coker which is operated under turbulent flow conditions. Here the wood sugars and lignins present in the liquor are coked. The total effluent from the coker consisting of water in liquid phase, suspended particulate coke l8 and a small amount of gas passes through a pressure reducing valve 4 to release a small amount of gas and then to the heat exchanger 2 where it is in heat exchange with the spent sulfite liquor feed. It should be noted here that this arrangement allows for the transfer of the heat content of both the coke and the liquid effluent to the incoming feed stream. The coker effluent, after heat exchange, drops into the vessel 5 which is provided with a trap tray 20 as shown. The coke settles to the bottom of vessel 5 and cokefree, cooled coker effluent is withdrawn through line 22 and pressure reducing valve 7 where the pressure is reduced to atmospheric. Any gases emerging from pressure reducing valve 7 can be conveyed to the boiler through a line not shown in the drawing. The cooled coker effluent at atmospheric pressure (optionally after NH addition through line 22) is passed to an absorber 8 where S is scrubbed from the relatively cool flue gas (at around 300F). A clean cool flue gas is produced and sent to the stack. The liquid from the absorber (now warm pulping liquor) is sent (after fortification, if necessary, with the usual pulping chemicals) to the wood digester 10 through line 9. In the digester 10 wood chips are reduced to wood pulp and spent sulfite liquor. The gases produced will contain H 8, methyl mercaptan, dimethyl sulfide etc. and are released through line 11 and along with the gases produced from blow vessel 12 are sent to boiler 15 to burn the sulfur compounds to S0 In the blow vessel 12, wood pulp is separated from the spent sulfite liquor which is recycled through line 13 as shown.

Wet coke from the separator vessel 5 is blown periodically by the pressure of the system as a dense slurry into the kiln 14. In the kiln the wet coke is contacted countercurrently with hot flue gas from the boiler 15 and emerges from the kiln as dry coke which is used to fuel the boiler and the coker heater. This contact at the same time cools down the flue gas for the S0 absorption step which is desirable. Process steam and hot flue gas are produced in the boiler 15 and all sulfur compounds are burned to S0 The flue gas then passes as a heat source to the kiln 14 and eventually to the absorber 8 where the S0 is recovered by scrubbing with cool coker effluent to reconstitute the wood digestion liquor.

The coker produced contains 6-7 percent sulfur and the sulfur is recycled for reasons of economies. In other metal-based (Na, Ca, Mg) sulfite processes sulfur recovery is also necessary and in those cases any metalbased chemicals adhering to the coke would be recovered as a result from the boiler. This is true also of the kraft" pulping which utilizes NaOH and Na s as pulping chemicals and the Soda process which uses NaOH. In the case of metal-based pulping processes, the smelt would be properly treated for recycle to the wood digestion portion of the process.

The process as described is most useful in the wood pulping industry where rather formidable problems exist with respect to both air and water pollution and also where it is necessary to recover chemicals both for reasons of economy as well as air and water pollution. The disclosed process allows for maximum recycle of water and pulping chemicals and essentially eliminates odor at least from the digester portion of a wood pulping plant. Since pulp and paper mills are heat deficient, the heat produced as process steam is welcome.

In the process, carbonization is substantially complete within a period of about 0.5 to 10 minutes. In a preferred embodiment of this invention, the outlet temperature of treated liquid leaving the tubular coking zone is maintained within the range of about 550 to 650F. The waste liquid undergoing treatment is maintained at elevated temperatures above 450F for a sufficient period of time, e.g. one minute, to obtain substantially complete carbonization of organic compounds contained therein to water insoluble solids or coke.

The coker used here is a tubular reactor. The flow of liquid in tubular reactor may be laminar or turbulent and for a given tube reactor the type of flow depends on the quantity of fluid flowing through the reactor. Turbulent flow is most desirable for coking as coke deposition on the reactor walls is minimized, thus assuring long operational life of the tubular reactor system. A useful relationship that substantially indicates the type of flow in a tube is the Reynolds number. The Reynolds number is defined by the relation:

N Reynolds number D= lnside diameter of tube, ft.

V= Average linear velocity, ft/sec.

e Fluid density, lb/cu. ft.

n Fluid viscosity, lb/(ft) (sec) Preferably Reynolds numbers above 4000 are maintained in the tubular reactor. It is preferable that the tubes in the heater be sized to insure turbulent flow conditions and optimize heat transfer.

While the present process has been described mainly with respect to its application to the treatment of wood pulping liquor, it will be apparent to those skilled in the art that the principles thereof are of wider applicability. Such applicability extends generally to any process in which process water must be purchased from a water supply and where the spent process water must be rejected to a waste disposal system or receiving stream.

Such processes include, for example, the treatment of canning plant wastes, citrus wastes, potato wastes and the like. More specifically, canning processes are usually heat deficient since heat is required for blanching, coking, etc. and accordingly these processes would also benefit from the heat recovery step of the present process by burning coke in a boiler to produce steam to satisfy the heat requirements of the canning plant. Any coke which is left can be used for other purposes such as soil conditioning.

What is claimed is:

1. Process for the treatment of a spent wood pulping liquor comprising coking same in a tubular coking zone under turbulent flow conditions characterized by a Reynolds number greater than 4000 for about 0.5 to 10 minutes at about 550F to 650F in the liquid phase to produce an effluent consisting of particulate coke suspended in water; bringing said effluent including said coke in heat exchange relationship with said liquor whereby the heat content of both said coke and said water is transferred to said liquor, then separating said coke from said resulting cooled effluent, drying said coke with hot flue gas containing S0 burning said dried coke to produce process steam and SO -rich flue gas, contacting said flue gas with said cooled effluent to recover S0 therefrom, thereby forming additional pulping liquor and clean cool flue gas.

2. Process as defined in claim 1 wherein said additional pulping liquor is passed on to a pulping area where wood pulp and sulfur-containing gases are produced, said gases being burned with said coke to produce additional flue gas.

3. Process as defined in claim 1, wherein said liquor is spent sulfite liquor.

4. Process as defined in claim 1, wherein said coke, upon being separated from said cooled effluent is discharged to a kilning zone for drying by hot flue gases produced by burning previously dried coke, thereby also cooling said hot flue gases.

5. Process as defined in claim 1, wherein said effluent is de-pressurized to about atmospheric pressure before being heat exchanged with said liquor.

6. Process as defined in claim 1, wherein ammonia is added to said cooled coker effluent.

7. Process as defined in claim 1 wherein pulping chemicals are added to said additional pulping liquor to fortify same. 

1. Process for the treatment of a spent wood pulping liquor comprising coking same in a tubular coking zone under turbulent flow conditions characterized by a Reynolds number greater than 4000 for about 0.5 to 10 minutes at about 550*F to 650*F in the liquid phase to produce an effluent consisting of particulate coke suspended in water; bringing said effluent including said coke in heat exchange relationship with said liquor whereby the heat content of both said coke and said water is transferred to said liquor, then separating said coke from said resulting cooled effluent, drying said coke with hot flue gas containing SO2, burning said dried coke to produce process steam and SO2-rich flue gas, contacting said flue gas with said cooled effluent to recover SO2 therefrom, thereby forming additional pulping liquor and clean cool flue gas.
 2. Process as defined in claim 1 wherein said additional pulping liquor is passed on to a pulping area where wood pulp and sulfur-containing gases are produced, said gases being burned with said coke to produce additional flue gas.
 3. Process as defined in claim 1, wherein said liquor is spent sulfite liquor.
 4. Process as defined in claim 1, wherein said coke, upon being separated from said cooled effluent is discharged to a kilning zone for drying by hot flue gases produced by burning previously dried coke, thereby also cooling said hot flue gases.
 5. Process as defined in claim 1, wherein said effluent is de-pressurized to about atmospheric pressure before being heat exchanged with said liquor.
 7. Process as defined in claim 1 wherein pulping chemicals are added to said additional pulping liquor to fortify same. 